According to GAT (PRN 4699) Carn Arthur, a possible Crannog, can be found a little beyond the (current) south-western end of Llyn Mwyngil at SH71000900 – that is where the Afon Dysynni makes a loop near a weir:
‘Stony mound.....referred to in a reliable C19th account as being visible at low water and a good place to fish’. Sources: Peterson, R. & Roberts, J. G. , 1989 , Archaeology in Wales.
Haven’t seen it myself.